The measuring devices, the measuring apparatuses or also the sensors of process automation technology monitor or measure process variables of media, which, depending on the type of process—e.g. the synthesizing of a medium—are subject to greater changes in relatively less time. Such dynamic processes require that the time intervals between individual measured values should be as small as possible. Thus, measurement should be as often as possible, in order to obtain an appropriate time resolution. For this, however, there is the problem of the energy requirement for the measurements, wherein a higher accuracy of measurement is usually associated with an increased energy requirement. In the case of 4 to 20 mA measuring devices, the energy is, in principle, limited. Depending on the embodiment, however, also components—e.g. power supplies—for higher powers are correspondingly more complicated or also costly. Thus, it is quite difficult to increase the number of the measurement data or measurement points. Another limiting factor is the physical measuring principle, which is applied and which, thus, also provides a lower limit for the time resolution. It can also occur that a higher time resolution is not continually required, because e.g. the measured process is, at times, steady. The need can thus be different, depending on process conditions.
Furthermore, the measuring device also requires energy for representation of the measured value on a display or for servicing and/or parameter input via a service unit.
The trend in measurements technology is to embody measuring devices such that they work as energy efficiently as possible: Four-wire measuring devices are being replaced in increasing measure by two-wire measuring devices. In the case of two-wire measuring devices—in contrast to four-wire measuring devices energy supply and measured value transmission occur via one and the same line-pair.
Two-wire measuring devices must be so designed relative to their power requirement that they always can be operated with the currently available power. Known from European Patent, EP 1 301 914 B1 is a two-wire microwave measuring device with clocked measuring rate. In the case of this known measuring device, the measuring rate is optimized such that a next measuring is started only when sufficient energy is available for such.
Described in Published International Application, WO 2004 046 6658 A1 is an ultrasonic, flow measuring device with a control/evaluation unit, which has a number of components, of which at least one has a high power consumption. This component, e.g. an amplifier, an ND converter, a microprocessor, is operated intermittently in rest and measuring phases.
Known from U.S. Pat. No. 6,014,100 is a two-wire radar measuring device with constant clocking of the measuring rate, which turns off at the worst or most unfavorable acceptable case (worst case) and works at the lower power limit: The constant clock rate of measurement is so designed over the total measuring range that in the case of minimum available power, thus in the case of a 4 mA signal, there is still sufficient power for the next measuring.
Of course, it is also in the case of this known solution problematic, how to provide customers with supplemental features in the servicing of the measuring device and the evaluation of the measured value. Moreover, a two-wire measuring device with constant measuring rate has, compared with a measuring device with variable measuring rate, the disadvantage that it has over almost the entire measuring range a lesser accuracy of measurement.
Besides two-wire measuring devices, which have per unit time only a limited amount of available energy, appearing increasingly in measurements technology are energy autarkic measuring devices. The terminology ‘energy autarkic measuring devices’ means measuring devices, which have no wired lines to a remote energy source of any kind. Energy autarkic measuring devices are supplied with energy e.g. via at least one battery, via radio, via solar cells or via fuel cells. In order that the duration of operation, e.g. life of the battery, be as high as possible, the measuring devices should be so embodied that they have a power consumption, which is as small as possible.
Due to the limited available energy, it is often difficult to offer customers two-wire measuring devices with so-called supplemental features. A classic example of a supplemental feature of measuring devices is an illuminated display, a touch-screen for simpler, intuitive input of parameter data or for intuitive servicing of the device.